1. Field of the Invention
This invention relates to the incorporation of carbon black, usually together with other fillers, in a polymeric material such as natural and synthetic rubber as part of the procedure of compounding.
2. Description of the Related Art
As has been known for many years, natural or synthetic rubber must be subjected to intensive working to make it softer and more pliable prior to vulcanisation. It has also been known for many years that to improve its characteristics, especially its tensile strength, and make it suitable for the manufacture of articles such as pneumatic tires, fillers must be incorporated and dispersed in the mix, the principal filler in terms of the proportion by weight utilised and the most important in terms of its effect on the characteristics of the compound being carbon black.
As manufactured particulate carbon black takes the form of groups of fused particles known as "primary aggregate". The groups are also more loosely interconnected in chains referred to as the "structure" of the filler. Successful incorporation requires that as high a proportion as possible of the voids existing between the particles of a group become filled with polymer and successful blending requires that the chains are at least to some extent broken up so as to achieve as even a dispersion as possible of the groups, or broken chains, within the body of polymer. At the same time the treatment should not be such as to destroy a high proportion of the groupings of particles, because the fusion or welds between these are believed to provide the necessary reinforcement of the compound.
Hitherto the processes of filler incorporation, blending and homogenising, and mixing or masticating have been carried out simultaneously by subjecting the heterogeneous mixture of polymer particles, carbon black and any other desired fillers to intensive working e.g. by passing it between opposed, counter-rotating rollers of a mill or by forcing it through specially shaped screws. The intensive working of the mixture generates considerable heat--typically in excess of 170 degrees centigrade--sufficient to plasticise the polymer. The perceived solution to poor incorporation or dispersion is to repeat the mixing process, the same batch of mixture being objected repeatedly to intensive working, or to extend the mixing screw so that the mixture is subjected to a lengthier mixing procedure. This of course leads to high energy consumption.